The present invention relates to the transfer of articles, such as semiconductor wafers, and more particularly to an improved end effector for handling and transferring very thin semiconductor wafers.
The use of robot arms is a well established manufacturing expedient in applications where human handling is inefficient and/or undesired. For example, in the semiconductor arts robot arms are used to handle wafers during various process steps. Such process steps include those which occur in a reaction chamber, e.g. etching, deposition, passivation, etc., where a sealed environment must be maintained to limit the likelihood of contamination and to ensure that various specific processing conditions are provided.
Current practice includes the use of an end effector (also known as a robot blade or carrier) attached to robot arms to load semiconductor wafers from a loading port into various processing ports within a multiple chamber reaction system. The robot arms are then employed to retrieve the wafer from a particular port after processing within an associated process chamber. The wafer is then shuttled by the robot arms to a next port for additional processing. When all processing within the reaction system is complete, the robot arm returns the semiconductor wafer to the loading port and a next wafer is placed into the system by the robot arm for processing. Typically, a stack of several semiconductor wafers is handled in this manner during each process run.
Currently, a conventional robot end effector is used to transfer wafers having a thickness of between about 0.020" and 0.030". Wafers having a normal thickness in this range are generally flat in that they typically do not exhibit more than about 0.005" of bowing. Furthermore, such wafers have sufficient weight to preclude slipping from the end effector as it is swung or moved between process locations attached to a wafer handling chamber.
However, under some circumstances, it is desirable to have a very thin wafer in the range of 0.005" to 0.010". For example, very thin wafers are better heat conductors than wafers of normal thickness. The thin wafer is formed by removing all the excess silicon from the back side of a wafer of normal thickness after the devices have been placed on the front side. The wafer is preferably thinned out by a process known in the art as "backlapping", whereby successive passes are made across the backside of the wafer with a tool until the desired thinness is achieved. Once the wafer is thinned to about 0.005", the backside is coated with gold, which results in a thin wafer with very good heat conductive properties.
Thinning of the wafer can cause problems, however. As the wafer is thinned, the materials forming the devices on the front side of the wafer induce stress and strain on the wafer causing it to bow. In many cases, this bowing is quite extreme. For example, wafers that have been reduced to a thickness of about 0.005" typically exhibit bowing of 0.075", and in extreme cases exhibit bowing of 0.120" or more. Several problems result from the use of prior art end effectors for transfer of these thin, bowed wafers. Because the thin wafers are bowed like a potato chip, the wafer does not sit flat on the end effector. Unless some mechanism is provided to hold the wafer in place, it may become displaced from the end effector during movement of the end effector because the wafer does not have sufficient contact with the end effector. The thin wafer also does not have sufficient weight to prevent it from being displaced from the end effector during end effector movement without some means for holding the wafer in place. Furthermore, as the thin wafers sit in the wafer cassette awaiting capture by the end effector, the bowing of the wafers causes the space between adjacent wafers to be significantly reduced. Use of prior art end effectors results in breakage of a number of wafers because the end effector cannot pass through the narrow space between adjacent bowed wafers in the wafer cassette.